Diodes function ideally as “one-way valves” in electronic circuits, allowing current to pass unhindered in one direction and blocking current in the opposite direction. Solid-state diodes are generally constructed from semiconductor crystals with different layers of the crystal having different electrical properties.
The most basic semiconductor diode is formed by adjoining two semiconductor crystals: one, called a P-type, that has been doped with atoms so that the majority of charge carriers are mobile positive holes, and the other, called a N-type, that has been doped with atoms so that the majority of charge carriers are negative electrons. This so-called “P-N junction” is connected to the rest of the circuit by attaching an anode material to the P-type semiconductor and a cathode material to the N-type semiconductor.
More complicated diodes can be constructed by having one or more layers of N-type semiconductor and/or one or more layers of P-type semiconductor, with different layers having different amounts of mobile charge carriers. In the case of a Schottky diode, the P-type material is omitted entirely with the anode connected directly to the N-type semiconductor.
An ideal diode will allow current to flow from the anode side to the cathode side with no drop in potential, and hence no loss of electrical energy. When current is flowing in this direction the diode is said to be in the “forward biased state.” An ideal diode will also completely block current flowing in the opposite direction regardless of the applied voltage, which is called the “reverse biased voltage.” An ideal diode will also be able to switch between these two operational modes instantaneously.
However, diodes do not function ideally. First, there is usually some small drop in potential when the diode is in the forward biased state. That is, there is some unwanted resistance to the free flow of current in the forward biased state, called “parasitic resistance.” Depending on the intended application of the diode, the losses due to the parasitic resistance may be large and may adversely affect components downstream from the diode.
Diodes also do not completely block current in the reverse biased state. The small current that flows in the opposite direction during reverse bias is called a “leakage current” and it often increases as the reverse bias voltage increases. Eventually, the leakage current increases dramatically when the reverse bias voltage reaches some rated breakdown voltage. Diodes with a leakage current that does not increase with increasing reverse bias voltage are referred to as having a “flat leakage current.”
Diodes also take time to switch between the on state and the off state. When conducting, the diode is flooded with electrons and holes moving in opposite directions. In order for the diode to transition to the off state this excess “stored charge” needs to be removed from the diode during what is called the “reverse recovery time.”